Many of us convert our garage to use as a playroom, studio home office, bedroom etc. This is relatively easily done and unless it is a permanent change you do not need planning permission or building regulation approval. Unfortunately garage are often built as an external storage area without insulation and is generally very cold and the external walls are often just single skin brick.
WALL and CEILING – the simplest thing to do is to use Thermaline Super, plasterboard with insulation on the back and stick this with nailable plugs and Dri Wall adhesive. This will provide an internal decoration as well as thermal insulation. Additionally fitted correctly the foil will prevent the ingress of moisture through cold bridging.
FLOOR (usually concrete) - a quick way is to lay XChip flooring which is a tongue and groove floor with high grade insulation on the back. As it is tongue and groove, you don't need to fix to the floor, the joint system combined with the weight will provide a stable floor as long as the main slab is level.
DOOR – the garage door can be replaced with a window but this usually requires planning permission and the building of a lower wall. One solution is to create a metal or wood stud and affix Thermaline Plus plasterboard – the door will be visible from the outside but not the inside. Prior to fixing the plasterboard you can also seal the door with mastic and fill the gap with Rockwool Flexi or Knauf Earthwool Combi 44.
Advice is given in this blog is given in good faith but every situation is different and everyone should seek expert advice. Councils operate a differing interpretation of building regulations and you should check that you comply fully. This is your sole responsibility.